Contamination of aqueous liquids with arsenic is a significant problem: e.g. natural groundwater in some locations, effluents from some mining operations, and wastewaters at certain industrial sites, have toxic levels of this element. Environmental authorities in Canada and United States currently have set the maximum level of arsenic in potable water at 50 ppb while the maximum level for dischargeable water is set at 50 ppb. Groundwaters at some locations in North America contain arsenic at levels up to about 100,000 ppb. Certain mining leachates and effluents from certain industrial and agricultural operations contain arsenic at concentrations as high as several hundred ppm.
There are several techniques currently used for the removal of arsenic from water, leachates or affluents.
a. Chemical precipitation
This method is based on the addition of chemical reagents to arsenic-contaminated water resulting in the formation of insoluble arsenic compounds. This technique is presently used in most mining leachate treatment plants and is effective for water contaminated with high levels of arsenic. The main disadvantage of this technique is its inability to reach the low concentrations of arsenic set out by the Environmental authorities. This treated water would have to be diluted with fresh or other treated water in order to achieve discharge limits. Several hundreds of ppb's is a typical residual concentration after treatment.
b. Ion exchange on polymeric resins
This technique is satisfactory only in the absence of higher concentrations of background salts, especially sulphates. Normally, groundwater has a relatively high level of inorganic substances resulting in the poor selectivity of the resins for arsenic. As a result, this method does not find broad industrial application for arsenic removal.
c. Filtration
This technique can be successfully used only if arsenic is present in an insoluble form, otherwise the effectiveness of this method is low.
d. Adsorption on inorganic adsorbents
It has been documented that the sorption of arsenic by inorganic adsorbents, especially activated alumina, can be very effective. The residual concentration of arsenic can be as low as several ppb. This technique is less sensitive than ion exchange on polymeric resins in the presence of larger quantities of background salts. It can, therefore, be used for the treatment of highly mineralized groundwater and mining leachates. This process, however, is relatively slow due to the slow diffusion of arsenic ions inside the granules of the adsorbent. It normally takes one or more days to achieve saturation of the adsorbent used.
It must be concluded that none of the existing technologies used for arsenic removal is ideal and universally applicable. Improvement of existing methods and the development of new techniques are, therefore, an environmental necessity.
Typical references describing the alumina adsorption currently used are the following.
Report EPA-600/2-83-107, Oct. 1983 US-EPA) E. Rosenblum & D. Clifford "The Equilibrium Arsenic Capacity of Activated Alumina" reviews various arsenic removal techniques (from water) and describes various tests using granular alumina (300-600 micrometers) in both batch and column modes. In a batch test using 3.33 g alumina per liter, almost one day was needed for removal of arsenic from about 4.5 ppm to the 50 ppb level (Table E-1 Appendix E).
U.S. Pat. No. 4,923,843 May 8, 1990 E. K. Saforo et al mentions using small alumina particles in the formation of a larger cementitious composite with activated carbon.
U.S. Pat. No. 5,118,655 Jun. 2, 1992 P. M. Pedersen et al describes the use of activated alumina of 50 to 100 mesh size to form a composite with active carbon and silicates, the composite being used in a canister or cartridge for water treatment.
It is evident that in the prior art where alumina adsorbent was used in water treatment it was used in the form of granules larger than 48 mesh (300 micrometers) size or as a component in forming a composite e.g. a larger granular composite or in a multicomponent medium. The prior art avoided using fine particles of alumina per se in water treatment, including treatment for arsenic removal.
While arsenic removal using granular alumina is reasonably effective it has the disadvantage of long treatment time with relatively large hold-up volumes. We have found it possible to significantly decrease the treatment times and thereby increase the overall efficiency of this process.